Subsystem: Inorganic sulfur (sulfate) assimilation
Christian Rückert, International NRW Graduate School in Bioinformatics and Genome Research,
Institute for Genome Research, Bielefeld University, Bielefeld, Germany
Sulfur is required for the biosynthesis of several essential compounds like amino acids (cysteine and methionine), vitamins (biotin,
thiamin), and prosthetic groups (Fe-S clusters) in all organisms. In order to synthesize these compounds, the sulfur has usually to be in a
reduced state, most commonly as (hydrogen) sulfide. In the absence of an environmental supply of reduced sulfur moieties (e.g. a black
smoker or another organism), organisms have to reduce the needed sulfur themselves. In many microorganisms this function is
performed by a very common pathway for assimilatory sulfate reduction, leading from sulfate to sulfide, which is then incorporated in
various sulfur containing metabolites (Fig. 1).
For each of the reaction steps there seem to exist at least two functional variants (Fig. 2) that appear to be only weakly correlated with
each other, so that almost any combination can be found (Fig. 3).
The first step in the pathway is the uptake of oxidized inorganic sulfur compounds, usually sulfate or thiosulfate. Several transporters
are known to be involved in this, like the ABC-type transporter Sbp CysAWPT in Escherichia coli  or the Pit-type permease (SulP) in
Bacillus subtilis . Following uptake, intracellular sulfate is activated by adenylylation, yielding adenosine phosphosulfate (APS). Two
different enzyme families of sulfate adenylyltransferase are commonly involved in this reaction: a heteromeric form (SAT1+2, usually
called CysDN) known from E. coli; and the homomeric form (DSAT) described for example in B. subtilis , which is usually used in
dissimilatory sulfate reduction . The next step is the reduction of the activated sulfate. APS is either phosphorylated to
phosphoadenosine phosphosulfate (PAPS) by APS kinase (ASK, CysC) and subsequently reduced to sulfite by PAPS reductase (PAPSR,
CysH) or converted directly by APS reductase (APSR, also called CysH). Which way is used in particular is hard to determine by
sequence similarity alone as APS and PAPS reductases belong to the same protein family and APS reductase has been shown to also act
on PAPS [4,5,6]. If not verified experimentally, it should therefore be assumed that APS reductase is bifunctional in an organism if APS
kinase is also present. The last step of the pathway is the conversion of sulfite to sulfide. In E. coli and B. subtilis this step is catalyzed
by a heteromeric form of sulfite reductase (SIR FP+HP, CysIJ), using NADPH directly as an electron donor . Interestingly, there are a
lot of organisms where only the hemoprotein subunit (SIR HP) or a protein more similar to the ferredoxin-dependent sulfite and nitrite
reductases known from plants is present. In the latter case, electrons for sulfite/nitrite reductase are derived either from the photosystem
I or, in non-photosynthetic tissues, from NADPH . These electrons are then transferred via an ferredoxin—NADPH reductase onto a
ferredoxin that in turn delivers them to the homomeric form of sulfite reductase.
At least one functional variant for each of the four steps leading from extracellular sulfate to intracellular sulfide was identified in
about 80 bacterial strains and species using SEED analytical tools. More importantly, two novel hypothetical variants were predicted for
each sulfate uptake and reduction of sulfite to sulfide (see next page), delivering new testable targets for functional genomics (Fig. 2).
Using SEED to identify “missing genes” and to predict novel functional
variants in Sulfur assimilation pathway
Several organisms lack a clear homologue of the ABC-type sulfate/thiosulfate transporter known from E.coli or the Pit-type sulfate
permease found, e.g., in B. subtilis. Based on co-occurrence two possible alternatives could be found:
1. A gene encoding a putative permease (CysZ, 7) is clustered with genes involved in sulfate reduction in two corynebacterial species
(Fig. 3, boxed in dark green, and Fig. 5; identifiable by a consecutive numbering in the species row). This permease might therefore
be involved in sulfate uptake. The experimental verification/falsification of this assumption is currently under way in our group.
2. Genes encoding an ABC-type transporter of unknown specificity are clustered with genes involved in sulfate reduction in at least
four bacterial species (Fig. 3, light green box).
Reduction of sulfite to sulfide
A significant number (about 40 out of 80 in total) of bacterial species currently present in the subsystem lack the flavoprotein subunit of
sulfite reductase (SIR FP, 18) known from E.coli and B. subtilis. Using SEED two possible alternatives were identified in this case as
1. In the genomes of more than 40 organisms (25 of which are currently present in the subsystem), SIR FP seems to be replaced by a
yet uncharacterized oxidoreductase (SIR FP2, 19; see Fig. 3, highlighted in bold red) which is clustered with the hemoprotein
subunit of sulfite reductase (SIR HP, 20) (Fig. 4, boxed in red). This variant can be found, e.g., in Sinorhizobium meliloti and
2. A second variant resembling the system found in plants can be found in the bacterial order of the Actinomycetales (Fig. 3, boxed in
orange, and Fig. 5) but cannot be found in other bacteria with exception of the Deinococcales. It consists of a ferredoxin-dependent
sulfite reductase (SIR FDX, 22), a ferredoxin—NADP(+) reductase (FPR, 23) and either a ferredoxin (FDX, 24) or a small,
ferredoxin-like protein (CysX, 25).
An interesting observation is the possible functional coupling and even fusions of FPR, FDX, and CysX detected in SEED: In some
organisms (e.g., M. tuberculosis and N. farcinica) fusion proteins of FDX and FPR are found, identifiable by the same number in
both columns (Fig. 5, highlighted in bold red). In all Actinomyetales present in the subsystem that lack this fusion (e.g. in
C. efficiens), a small protein distantly related to ferredoxins (CysX) is clustered with other genes involved in sulfate reduction.
Apparent deviations in C. glutamicum and T. fusca (Fig. 5, marked with arrows) are due to miscalling of this small ORF by an
automatic software - in both cases a clear CysX homologue exists.
The novel genes for both novel functional variants for the reduction of sulfite to sulfide are currently under study in our group to
elucidate whether our predictions are correct.
Fig. 1: Subsystem diagram illustrating functional variants in assimilatory
sulfate reduction pathway
Fig. 2: Functional roles and their assignment to functional variants
The list of functional roles included in the subsystem Inorganic sulfur assimilation in SEED. Potential
novel functional variants identified with SEED are highlighted.
Fig. 3: A snapshot of the subsystem spreadsheet illustrating various combinations of functional
variants in different reaction steps.
Fig. 4: Positional coupling of the hemoprotein subunit of sulfite reductase (SIR
HP) with an uncharacterized oxidoreductase (SIR FP2)
Fig. 5: Conservation of the ferredoxin-dependent sulfite reduction in different
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